1. Field of the Invention
The present invention relates to a device for driving a thin film transistor liquid crystal display (TFT-LCD), which more specifically can reduce power consumption by eliminating large power consumption elements such as analog switches, operational amplifiers and a push-pull circuits.
2. Description of the Related Art
In order to continue the current trend toward light weight and compact electronic products, research and development in liquid crystal displays (LCDs) as replacements to cathode ray tubes (CRT) has been undertaken.
LCDs or flat-panel displays provide many advantages, including light weight and thin volume. A further and more significant advantage is that LCDs permit adapting an LSI driver because LCDs can be driven by low voltage and power. For these reasons, most manufacturers have invested in technical development and practical use of LCDs.
A TFT-LCD is a LCD which combines each pixel with a thin film transistor. These transistors are constructed in such a way that amorphous silicon is applied to a glass substrate by thin film techniques, and a twisted nematic liquid crystal is used as a liquid crystal material.
TFT-LCDs are advantageous in that no crosstalk is generated, because only one pixel can be turned on when receiving a signal from the transistor. It is also advantageous in that the entire display can be maintained during a non-selection time because each pixel has a thin film capacitor which stores voltage.
A typical TFT-LCD is shown in FIGS. 1 to 5, and is discussed in greater detail below.
FIG. 1 shows a pixel circuit diagram of a conventional gate type TFT-LCD. As shown in FIG. 1, a conventional pixel circuit includes a switching transistor TFT, a liquid crystal Cic connected between the drain terminal of the switching transistor TFT and a common electrode, and a capacitor Cst connected between the drain terminal of the switching transistor TFT and a gate terminal of another switching transistor TFT.
The main feature of this circuit is that one electrode of the capacitor Cst is connected to a gate terminal of another switching transistor TFT.
To drive this circuit, an AC common driving method is commonly used. In this method, a common voltage signal Vcom having an AC waveform is applied to the common electrode connected to the liquid crystal Clc, and gate-on/-off voltage signals Von/Voff for periodically turning-on/-off the switching transistor TFT are applied to the gate terminal of the switching transistor TFT.
In this case, the common voltage signal Vcom should be in the same phase as that of the gate-on voltage signal and the gate-off voltage signal for at least two reasons.
First, the capacitor Cst and the liquid crystal Clc are connected to the common voltage signal Vcom and the gate-on voltage signal Voff. Here, if the common voltage signal does not have the same voltage and amplitude as those of the gate-off voltage signal Voff, the potential difference between Von and Voff, varying upon the time, leads to leakage current from the capacitor Cst and the liquid crystal Clc. This increases power consumption, while showing unstable display characteristics.
Second, in an LCD, a kick back voltage should be maintained uniformly. As the kick back voltage is proportional to Von-.vertline.Voff.vertline., Von-.vertline.Voff.vertline.should be maintained uniformly. However, if the kick back voltage is not uniform, abnormal display characteristics may be generated.
A circuit such as the one shown in FIG. 2 is typically used in conventional devices to generate the gate-on voltage signal Von. This circuit will now be described.
FIG. 2, is a detailed circuit diagram of a gate-on voltage generator employed in a conventional TFT-LCD. As illustrated in FIG. 2, a gate-on voltage generator includes an analog switching circuit AS having input terminals connected to a pair of switching signal lines POL, /POL, a pair of gate-on voltage signal lines Von1, Von2, and an input power voltage VEE. A non-inverting input terminal of an operational amplifier is connected to an output terminal of the analog switching circuit AS and an input terminal of a push-pull circuit is connected to an output terminal of the operational amplifier.
In the gate-on voltage generator, when a switching signal POL or /POL is applied to the analog switching circuit AS, the analog switching circuit AS alternately outputs the gate-on voltage signal corresponding to Von1 in the high state or Von2 in the low state.
The output signal from the analog switching circuit AS is applied to the operational amplifier and the push-pull circuit in order to divide the impedance, before outputting the gate-on voltage signal Von, independent of the load conditions, as illustrated in FIG. 3. In this case, Von1-Von2 should be equal to Vcom1-Vcom2 to uniformly maintain the kick back voltage.
Further, to output the gate-on voltage signal Von1 in high state, power voltage VEE should satisfy the following condition: EQU VEE&gt;Von1+V.sub.BE +Vos
where, V.sub.BE is an amount of lowered voltage at the push-pull circuit, and Vos is an offset voltage between a power voltage VEE and a maximum possible output voltage of the operational amplifier. In a typical TFT-LCD, the power voltage VEE is generated from a DC/DC converter.
However, this conventional gate-on generator of a TFT-LCD driving circuit has several problems. First, it is difficult to adjust the levels of the gate-on voltage signals Von1, Von2 to equalize with the amplitude of the common voltage signal Vcom. A more serious problem is that adjustment of the gate-on voltage signals Von1 and Von2 is required since each component of the TFT-LCD module may have a tolerance and, therefore, each module has a slightly different common voltage Vcom. This troublesome adjustment lowers the productivity during manufacturing.
Second, the power voltage VEE should be a considerably high voltage, typically above +20V, and total power consumption of the TFT-LCD is too large due to the analog switch AS, the operational amplifier, and the push-pull circuit.
Third, a DC/DC converter is required to generate the power voltage VEE, thereby increasing the production cost.
A conventional gate-off voltage generator for use in a TFT-LCD will now be described with reference to FIG. 4. As illustrated in FIG. 4, a conventional gate-off voltage generator includes an analog switching circuit AS having input terminals connected to a pair of switching signal line POL, /POL, a pair of gate-off voltage signal lines Voff1, Voff2, and a power voltage VSS. A non-inverting input terminal of an operational amplifier is connected to an output terminal of the analog switching circuit AS, and a push-pull circuit having its input terminal connected to an output of the operational amplifier.
Operation of this conventional gate-off voltage generator will now be described.
When the switching signal POL or /POL is applied to the analog switching circuit AS, the analog switching circuit AS alternately outputs a signal corresponding to the gate-off voltage signal Voff1 in high state or Voff2 in low state.
The output signal from the analog switching circuit AS is applied to the operational amplifier and the push-pull circuit in order to divide the impedance, before outputting the gate-off voltage signal Voff, independent of the load conditions, as illustrated in FIG. 5.
In general, the gate-off voltage level is within the range of -7V to -2V, because the turn-off voltage level of the TFT-LCD is below -2V.
However, this conventional gate-off generator has several problems. First, it is difficult to adjust the levels of the gate-off voltage signals Voff1, Voff2. In more detail, the gate-off voltage signals Voff1 in the high state and Voff2 in the low state should be equal to the amplitude of the common voltage signal Vcom, and therefore the gate-off voltage signals Voff1 and Voff2 should be adjusted one by one in response to the amplitude of the common voltage signal Vcom, within a certain tolerance. This difficult adjustment lowers manufacturing productivity.
Second, power consumption is considerably large. To generate an electric potential of -7V which is a level of the gate-off voltage signal Voff, a power below -7V is required for the analog switch, the operational amplifier and the push-pull circuit, which must be obtained from the DC/DC convertor. Although low power consumption is a necessity in portable LCDs, the entire power consumption of the conventional gate-off voltage generator increases since the analog switch, operational amplifier and push-pull circuit consume a considerably large amount of power and the power conversion efficiency of the DC/DC convertor is below 70%.